1. Field of the Invention
This invention relates to image sensor systems and, more particularly, to an improved design for digital pixel sensors.
2. Related Art
Digital photography is one of the most exciting technologies that have emerged in the past years. With the appropriate hardware and software (and a little knowledge), anyone can put the principles of digital photography to work. Digital cameras, for example, are on the cutting edge of digital photography. Recent product introductions, technological advancements, and price cuts, along with the emergence of email and the World Wide Web, have helped make digital cameras the hottest new category of consumer electronics products.
Digital cameras, however, do not work in the way that traditional film cameras do. In fact, they are more closely related to computer scanners, copiers, or fax machines. Most digital cameras use an image sensor or photosensitive device, such as charged-coupled device (CCD) or Complementary Metal-Oxide Semiconductor (CMOS) to sense a scene. The photosensitive device reacts to light reflected from the scene and can translate the strength of that reaction into electronic charging signals that are further digitized. By passing light through red, green, and blue filters, for example, the reaction can be gauged for each separate color spectrum. When the readings are combined and evaluated via software, the camera can determine the specific color of each segment of the picture. Because the image is actually a collection of numeric data, it can easily be downloaded into a computer and manipulated for more artistic effects.
Digital cameras, however, do not have the resolution attainable with conventional photography. While traditional film-based technology, limited only by the granularity of the chemically based film, typically has a resolution of tens of millions of pixels, image sensors for use in most commercially viable digital cameras acceptable to general consumers have a resolution of slightly more than a million pixels.
Furthermore, the dynamic range of digital image sensors is often not as broad as is capable with film-based conventional photography. This is especially true for CMOS image sensors which, in general, have lower dynamic ranges than do CCDs. FIG. 1 shows a block diagram of a digital image sensor 10 as disclosed in U.S. Pat. No. 5,461,425 to Fowler, et al. As is shown, an image sensor core 12 have a two-dimensional array of pixels 15, each having a light detecting elements (photodetectors or photosensors) coupled to a dedicated A/D converter which outputs a stream of bits representative of the analog output of the light detecting element. In other words, the image sensor by Fowler outputs directly digital image data. Not only does subsequent supporting circuitry become dramatically simplified, but also there are numerous advantages provided by this architecture in view of the traditional CMOS image sensors. The advantages include better control of operations of the image sensor and far better image qualities therefrom. However, adding a dedicated A/D converter to each of the light detecting elements could introduce some practical problems that may limit the deployment of such image sensors. One of the problems is that the image sensor core 12 is inevitably larger than it would be without the dedicated A/D converters. If an image sensor is desired to have millions of photodetectors thereon, there would be a large number of dedicated A/D converters, which could take a significant amount of space in the image sensor core. Larger image sensor cores typically mean high cost and low yield. There is therefore a need for new designs of digital image sensors that directly produce the digital image data. Further, the sensitivity of the photodetectors could be compromised when the photodetectors are needed to be smaller to accommodate the dedicated A/D converters in an image sensor of limited size. Under normal processing, the efficiency and sensitivity of the photosensitive portion of the pixel is decreased with decreased CMOS device size. Additionally, the photosensors suffer increasingly large leakage currents, and therefore become increasingly noisier, as feature sizes are decreased. This decreased sensitivity results in pixel devices with correspondingly lower dynamic ranges, resulting in the need for further supporting circuitry at the pixel to compensate. The increased amount of supporting circuitry at the pixel defeats the benefits of smaller device sizes for the chip. Therefore, there is another need for devices with CMOS photosensors having high efficiencies and sensitivity while maintaining small device sizes for supporting circuitry.
In accordance with the present invention, photosensors of an imaging sensor are constructed on a first substrate and supporting circuitry for the photosensors are constructed on a second substrate. The first substrate and the second substrate are then electrically coupled such that each of the photosensors on the first substrate are electrically coupled the supporting circuitry on the second substrate. In one embodiment, the first substrate and the second substrate are electrically coupled through bumping. In other words, bumps of conductive material (e.g., indium) are formed on a top surface of the second substrate, the bumps being electrically coupled to the supporting circuitry. Vias and similarly conducting bumps are constructed appropriately in the first substrate so that photosensors on the first substrate are electrically coupled to the bumps on the second substrate when the first substrate and the second substrate are aligned. In most embodiments, the first substrate is positioned in contact with the second substrate and bonded into place. In some embodiments, the first and second substrates can be produced using the same processing technology (e.g. CMOS) while in other embodiments different processing technologies can be utilized for producing the substrates.
In one embodiment, the imaging device includes an array of pixels. Each pixel includes a photosensor and pixel supporting circuitry for that photosensor. According to the present invention, the photosensor is formed on the first substrate and electrically coupled through a via and bumping technology to the pixel supporting circuitry on the second substrate. In another embodiment, multiplexing circuitry may also be formed on the first substrate and multiple photosensors can thus utilize the same bump in the second substrate to couple signals between photosensors on the first substrate with pixel support circuits on the second substrate.
Different embodiments of the invention may yield one or more of the following advantages and benefits. First, the size of an image sensor employing the present invention will not be enlarged significantly to accommodate the large number of pixel-level supporting circuits and therefore the sensitivity of the image sensor can be preserved. Second, the yield of semiconductor wafers in production of such an image sensor can be maintained as the size of the image sensor remains nearly unchanged. As a result, the cost of such image sensors is controlled and the popularity of the image sensors can be materialized.
These and further embodiments of the invention are discussed in more detail below with reference to the following figures. Other objects, benefits and advantages of embodiments of the invention are also discussed below.